VIIRS as an Arctic Nightlight

Published: April 17, 2013

Collage of images from VIIRS.

PHOTO: Courtesy of UAF, NOAA-NESDIS JPSS, and GOES-R

By Elizabeth O'Connell

During the winter in the Arctic it’s “night” almost all the time, but thanks to the new Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB) we no longer have to be in the dark about what’s going on with the weather. Here is a VIIRS/DNB image of the Alaska region on December 31, 2012 generated by GINA, Geographic Information Network of Alaska, at the University of Alaska Fairbanks. Although it looks like a typical daytime picture (with brightly reflecting clouds, sea ice, and snow cover contrasting against dark open waters of the north Pacific), the source of illumination in this case is the Moon.

Satellite image of Alaska Region

IMAGE: Courtesy of UAF, NOAA-NESDIS JPSS, and GOES-R

“The night is nowhere near as dark as most of us think. In fact, The Earth is never really dark,” said Steven Miller, an atmospheric scientist at Colorado State University. At night, city lights on earth map the urban areas, and show major population centers. But even away from human settlements, light still shines. There are wildfires, volcanoes, and oil and gas flares. The Aurora borealis is a light show across the polar skies. Moonlight reflects off water, snow, clouds, and deserts. Even the air and the oceans sometimes produce a detectable glow!

Or as former Alaskan Tom Bodett might say; Somewhere, a light has been left on and VIIRS can see it.

Scientists have been observing earthly night lights for over four decades via military satellites and astronaut photography. However, a new satellite, the Suomi National Polar-orbiting Partnership (NPP) carryies the cutting-edge technology VIIRS sensor launched in October 2011 by NASA and now managed operationally by the National Oceanic and Atmospheric Administration, (NOAA) has made the nocturnal view significantly clearer. “It’s a huge step forward,” said Christopher Elvidge, NOAA scientist/pioneer working with the Defense Meteorological Satellite Program (DMSP) for over 20 years. Tests between VIIRS/DNB and the heritage DMSP Operational Unescan System sensor (OLS—providing a version of low-light visible imagery) show DNB has substantially improved detection limits.

The DNB can observe dim light down to the scale of an isolated highway lamp or a fishing boat. This new technology has six times better spatial resolution than the OLS; 14 bit depth (16,348 levels of information) when it used to be 6 bit depth (64 levels), and the 21 other bands on VIIRS are multispectral, meaning it is far more capable of characterizing the environment. “The sum of these measurements gives us a global view of both the Earth’s natural environment and a portion of the human footprint within it,” said Miller. This photo/graph of North and South Korea and the Yellow Sea with fishing boats shows how VIIRS can visually depict political realities of the region. Specifically North Korea appears to be almost an island with very few lights except for Pyongyang, North Korea’s capital city. The fishing boats toe the line, constrained by politically-defined zones in the region.

Night view of North Korea and South Korea

IMAGE: By Seven Miller

Fishing Boats in the Yellow Sea

IMAGE: By Steven Miller

“Compared to the Lower 48 states, Alaska is different, and different often means things are tougher to do here,” said Eric Stevens, science liaison with GINA. “For example, the Alaskan winter is very dark, and how can you see clouds if it’s dark out? But now Alaska’s special circumstances are an advantage, because VIIRS DNB can use our surplus of winter moonlight to produce satellite pictures at night,” said Stevens. “Alaska’s density of radar coverage is vastly inferior to that in the lower 48. Also, the complex terrain in Alaska limits the reach of weather radars and other surface-based observations,” continued Stevens, “But the VIIRS satellite imagery is spatially comprehensive and helps fill in the gaps for weather forecasting.” “In the case of VIIRS, the difference is good,” said Stevens.

GINA Satellite Dish

PHOTO: Courtesy of UAF/By Todd Paris

VIIRS Satellite

PHOTO: Courtesy of NASA

GINA and VIIRS work well together. GINA, the satellite dish housed in the white globe on the University of Alaska Fairbanks campus, is a big ear listening for VIIRS. VIIRS’s orbits near the poles at an altitude of 512 miles, (for comparison the International Space Station’s altitude is between 205 and 256 miles above Earth’s surface), providing imagery across a 3000 kilometer wide swath. The high latitudes benefit greatly from the nature of this so-called ‘ polar orbit’ with GINA receiving a series of about 6-8 passes spaced apart by 100 minutes during two extended periods of the day. The Suomi-NPP satellite completes about 14.5 orbits in a 24 hour period and Alaska receives 7-8 times more imagery refreshes than a location near the equator. When Suomi-NPP comes within line-of-site of GINA’s X-Band Antenna, the digital data are captured and converted into useful images.

Collage of images from VIIRS

IMAGE: Courtesy of UAF, NOAA-NESDIS JPSS, and GOES-R

There are myriad benefits of DNB imagery in shedding light on the great darkness of the Alaskan winter. For example, VIIRS can see through many cirrus clouds because of the way clouds behave at visible light wave lengths. While these clouds appear optically thin to visible light, they are rather opaque at conventional infrared measurement channels—obscuring any features of interest below them--like sea ice structures of prime interest to the maritime community. “With conventional thermal band (infrared) measurements, cirrus clouds are opaque and block the earth they cover,” said Miller

Recently the VIIRS DNB captured incredible detail of the arctic ice cap, commonly thought of as a continuous sheet of stationary floating ice. Imagery acquired in February demonstrated the ice sheet is in fact a collection of smaller pieces shifting, cracking and grinding against one another.

Stevens, a meteorologist at the National Weather Service for 18 years and now GINA’s science liaison, emphasized how critical it is to know what the current weather is to be able to forecast into the future. “VIIRS is the latest and most modern tool available to accomplish that,” said Stevens.

Miller, Deputy Director of the Cooperative Institute for Research in the Atmosphere (CIRA), at Colorado State University will continue to focus on the many science pursuits of satellite remote sensing and the VIIRS DNB. Miller joins others in asking and answering these questions: How does the Earth/atmosphere operate as a system at time scales ranging from weather to climate?, How do we best observe the ‘vital signs’ of this system?, and What does this information really tell us?
“We have entered a new era, “ said Miller, “one where we can begin to literally probe into the darkness and better understand the processes of high latitude weather and climate. We realize that everything is connected as a system, almost like a living breathing organism, but the details of these connections are very elusive. We now have superior observation tools that can begin to shed new light on grand challenges of environmental science.”

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